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Forestry 485

Forestry 485. Lecture 3-3: Polyurethane and Epoxy Adhesives. Polyurethane Adhesives. Chemistry is same as for MDI Consists of two components: “A” side=isocyanate, containing MDI or TDI “B” side=polyol=hydroxyl-containing molecules

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Forestry 485

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  1. Forestry 485 Lecture 3-3: Polyurethane and Epoxy Adhesives

  2. Polyurethane Adhesives • Chemistry is same as for MDI • Consists of two components: • “A” side=isocyanate, containing MDI or TDI • “B” side=polyol=hydroxyl-containing molecules • “A” and “B” are kept separate physically or chemically (e.g., by emulsion) until use • Mixing of iso and polyol results in chemical reaction

  3. Polyurethane cure • When iso and polyol are combined, a “urethane bridge” or urethane chemical linkage is created: R-N=C=O + HO-R’ R-NH-C-OR’ Urethane “bridge”

  4. Polyurethane cure • Iso reacts readily with water and should also react with wood substrates • Reaction with moisture in air and/or from wood speeds cure process • Excess moisture may result in liberation of carbon dioxide, resulting in foaming of the adhesive (“blowing” reaction) • Cure/setup of adhesive is generally rapid

  5. Epoxy adhesives • 100% solids; no solvent loss upon cure, resulting in low shrinkage of adhesive • See p. 90 of Marra (Reading 3-3A) and class blackboard notes for example of chemical structure • Contains two components: Epoxy plus catalyst or reactive hardener • Forms an insoluble, infusible, thermoset resin

  6. Epoxy hardeners • Many classes of compounds used, depending on application, including: • Polyamines • Polyamides • Polysulfides • Urea resins • Phenolic resins • Acid anhydrides • Some of these compounds are toxic; ALWAYS read and follow label directions!

  7. Other components of epoxies • Fillers are used to increase gap-filling ability • Example materials include titanium dioxide and ferric oxide • Fillers may be up to 50% by weight • Epoxies are generally used for specialty applications due to high cost • Wood-epoxy bonds are not durable in long-term exterior exposure due to variation in dimensional change of substrate and adhesive; failure occurs at wood-epoxy interface.

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